This invention relates to sorptive carriers and methods for producing granular sorptive carriers. In one aspect, the invention relates to a substantially neutral and inert carrier for agricultural chemicals which carrier comprises a granule of calcium sulfate dihydrate having certain beneficial properties.
A number of solid materials are widely used as carriers for agricultural chemicals, such as insecticides, herbicides, fertilizers, and the like. The agricultural chemicals are combined with such a carrier for convenient dissemination by various distributor means.
In some types of agricultural carriers, the chemical or active ingredient contained therein is in solid form, usually as a powder or as small particles or granules, and is admixed with the carrier, the mixture then being formed into pellets. With other types of carriers, the carrier is in the form of particles or granules into which the active ingredient, in liquid form, is absorbed. With yet another type of carrier the active ingredient is adhered to the carrier surface.
Agricultural carrier material can be used in many forms, such as powder, particles, granules, or pellets. For ease of handling, and for other reasons, materials having a granule size in a range which would pass through a 20-mesh screen and be retained on a 60-mesh screen (U.S.A. Standard Sieve Series) are commonly used. With such size granules, it is important that the granules maintain their structural integrity and thus size during initial fabrication as well as during subsequent storage, marketing, and application. In many applications, it is important that the particles or granules be of a size that does not pass through the 60-mesh screen so as to reduce the probability that some of the particles or granules are so small as to form dust. It is also important that the particles maintain their size and condition so that they do not form dust, or turn to dust, owing to degradation during storage or use, or owing to general abrasion or attrition during manufacturing, handling, storing, transporting and application with mechanical devices to agricultural soil. Dust is objectionable because of the well known problems with dust spreading in the air and on persons and animals, and being inhaled by workers making or handling such carriers.
Many naturally-occurring mineral carriers that are used with agriculturally active ingredients, including certain types of pesticides, have a degree of surface acidity which varies depending upon the crystalline and molecular structure of the mineral. It is thought that the surface acidity arises as a result of a non-uniform distribution of electric charge in or on the surface of the mineral particles. A large number of electric charges may exist at certain areas on a surface of a mineral carrier particle and these are referred to as acid sites or electrophilic centers. The strength of these centers varies depending upon the composition of the surface and the degree of distortion in the structure which brings about the non-uniform distribution of the electrical surface charges. The surface acidity on a mineral carrier particle can affect the reactivity of that mineral particle with the agricultural chemical carried thereon. It is thought that the surface acidity, and specifically the acid centers, have a catalytic effect with respect to the decomposition of the particular chemical. It has been found that with some pesticidal chemicals, the catalytic activity of the acid sites, with respect to inducing or accelerating decomposition, can be much reduced by deactivation of the acid sites with certain organic or inorganic materials which preferentially share their electrons with the mineral to form a bond which is stronger than that which may be formed between the agricultural chemical and the acid center itself. The addition of any deactivator material, usually in amounts of up to 6 to 8 percent by weight of the carrier adds an undesirable cost to the formulation of the agricultural chemical-laden carrier. Thus, it would be desirable to provide a substantially neutral and inert carrier for agricultural chemicals, and especially for pesticides, which has little or no surface acidity and which preferably does not require the use of any deactivator material in conjunction therewith to inhibit decomposition of the active ingredient (the carried chemical) on the carrier.
An absorbent carrier for liquid chemicals should have a relatively high sorptivity, or a sorptivity which is at least high enough to prove commercially satisfactory.
In the case of absorbent materials, the porosity of the material is usually related to the absorbency characteristics of the material. Further, a generally low dry bulk density is usually a characteristic of the more absorbent materials. Generally, as particle or granule size increases, the bulk surface area decreases for a given number of granules. Since sorptivity is principally a surface phenomena and a function of the pore density within a surface, it would be desirable to provide a carrier granule having a size small enough to present a relatively high bulk surface area and having a pore density high enough such that the sorptivity is commercially satisfactory. Further, the size of the particle cannot be so small as to constitute a powder or dust which is objectionable for the reasons previously explained or so small or friable that during manufacture large quantities of the carrier cannot be made without an undesirable amount of small, dust size particles being concurrently produced.
In order that a granulated carrier function properly and not degrade through abrasion or attrition into dust under mechanical stress during manufacture, packaging, storing, shipping and use, the carrier granules must exhibit adequate mechanical strength. Thus, it would be desirable to provide a carrier granule which has relatively high mechanical strength or resistance to attrition.
It would also be desirable to provide a carrier which is relatively inert and inexpensive, and one which can be made from relatively plentiful natural substance. One such substance is gypsum which occurs naturally in a form having the chemical formula CaSO.sub.4.2H.sub.2 O. However, when naturally occurring gypsum is crushed into particles, the bulk density is about 65 to about 70 pounds per cubic foot. As discussed above, the higher the bulk density, the lower the sorptive capacity. Consequently, the sorptivity of such naturally occurring gypsum entities in the above-stated size range is relatively low, and in general is too low to provide a commercially acceptable sorptive carrier for agricultural chemicals in liquid form. However, the present invention provides relatively low-density gypsum granules that are eminently suitable as carriers for agricultural chemicals and a method for manufacturing these granules utilizing fluidized bed techniques under controlled conditions. Additionally, the low-density gypsum granules produced according to the present invention are useful as oil and grease absorbents, as absorbents for household pet toilets, and for similar applications.